DuraStar®
Processing Instructions for DS1000 Series
Introduction
DuraStar polymers afford the consumer brilliantly clear polymers as well
as excellent impact strength, chemical resistance, dimensional stability,
low shrinkage rates, and other enhanced physical property advantages.
These processing guidelines are given to help optimize these physical
properties and widen the processing window.
Drying
- Use a desiccant-type drying system with dry air at a minimum dew
point of –29ºC (–20ºF).
- Dry DuraStar polymer at 70º–75ºC (158º–167ºF) for 2 hours
minimum. Residence time in the dryers can be up to 24 hours. The
inlet air temperature needs to be controlled within ± 3ºC
(± 5ºF) throughout the drying cycle.
- The dryer should have sufficient airflow to ensure a uniform pellet
temperature throughout the dryer. A minimum of 3.7 M3/h airflow is
suggested for each kilogram of polymer processed per hour (1.0 cfm
per pound per hour of polymer processed).
Processing
Barrel and Melt Temperatures
- Processing at the lowest processing temperature and residence time in
the machine will maximize physical properties.
- Well-dried material is the key to shot-to-shot conformity. Engineering
materials tend to be more sensitive to degradation at their processing
temperatures due in part to the hygroscopic nature of the polymer.
- Normal processing temperatures are in the range of 235º to 254ºC
(455º to 490ºF) air shot. When molding parts at faster cycle times
utilizing larger barrel capacity, 50%–80% can be run at higher range of
the melt temperature. Contrary to this, when parts are molded with
long cycle times utilizing a minor amount of the barrel capacity,
10%–25%, the processor should strive to run the polymer at the lower
range of the proposed melt temperature.
- A flat temperature profile setting is normally used; i.e., a barrel with a
three-zone system might have settings as follows:
|
Rear Zone
|
240ºC (465ºF)
|
|
Center Zone
|
240ºC (465ºF)
|
Front Zone
|
240ºC (465ºF)
|
Nozzle Zone
|
240ºC (465ºF)
|
- Since each machine is different, the barrel set temperatures might need
to be set as much as 10º–20ºC (20º–40ºF)lower than the targeted
melt temperature because of shear heating. It is good practice to
determine the actual melt temperature using a pyrometer. Also, it is
important that the casting around the throat of the injection-molding
machine be cooled to provide optimum pickup of the material.
Mold Temperatures
- Mold temperatures ranging from 13º to 30ºC (55º to 84ºF) produce
the best parts.
- DuraStar polymers require colder molds than some other plastics, so
preparing cooling ahead of time pays dividends in cycle time and
processability. High mold temperatures even in small areas of the mold
can cause sticking. Ample mold-cooling channels, good cooling of pins
and thin steel areas, good cooling near hot spots such as sprues or hot
runners, insulating areas around hot runners, good water supply with
few flow restrictions, and thermolators for exact setting control of
water temperature all assist in generating fast cycling parts with good
surface appearance.
- With good cooling as outlined above, the cooling part of the cycle can
be minimized to a point where the part is solidified and easily ejected
while the larger diameter sprue is often still soft and rubbery.
- Additional cooling could be needed to prevent sprue sticking. Review
the mold construction guidelines for additional information.
Fill Speed
- Fill speeds used for DuraStar polymers are slower than for typical
plastics. Machines with fill-speed profile capability are recommended.
Where fill-speed profiling is available on a machine, starting the fill at a
very slow speed such as 13 mm (0.5 in.) per second for the first 5% to
15% of the shot, then increasing to 43 mm (1.7 in.) per second, and
then slowing to 23 mm (0.9 in.) per second is often successful. The
slower initial fill speed minimizes gate blush. Where direct sprue gating
into the part is used, a moderate to fast fill rate such as 38 to 56 mm
(1.5 to 2.2 in.) per second is suggested.
- Gate geometry is also very important to part appearance near the gate.
If the gate or runner has sharp corners or other nonstreamlined
features in the flow channel, these may need to be radiused to reduce
blush near the gate. Gate thickness as well as speed can influence gate
blush. Gate thickness less than 1.1 mm (0.045 in.) is not suggested for
most gate types.
Screw Speed (rpm)
- The rpm of the screw should be slowed to the minimum speed
necessary to recover the screw during part cooling and sit at the rear
position only 2 to 5 seconds before the mold opens. This minimizes
high-speed shear and tends to make the melt more uniform.
Pack and Hold
- Where direct sprue gating into the part is used, longer hold times in
combination with lower hold pressures might be necessary. If a void
develops at the base of the sprue, the sprue has a tendency to stick in
the mold, separating at the part. Packing out the void strengthens the
sprue so that it will release with the part. Having long hold times of
8 to 12 seconds and lower hold pressures of 34 to 52 MPa (5,000 to
7,500 psi)1 will feed material to the sprue to fill the void while not
overpacking the sprue. Overall cycle time does not have to be extended
if you decrease the cooling timer by the amount you raised the hold
timer. Sticking can also occur with a conventional runner at the
junction of the runner and sucker pin. Again, if the sprue sticks in the
mold, utilizing the same methodology will help eliminate the problem.
Cushion Size
- Cushion size should be at the absolute minimum to ensure the screw
does not hit bottom and the pack and hold pressures are getting into
the part. The cushion left at the end of the pack and hold is typically
5 to 10 mm (0.2 to 0.4 in.) depending on machine size and injection
speed. Larger cushions can add to hold time in the barrel and aggravate
degradation. If the screw continues to move forward at the end of the
shot after adequate time is given to come to a stop, it is a sign of a
leaking check valve, which may also cause short shots and shot-to-shot
variability.
Back Pressure
- Back pressure is usually minimum at about 10 MPa (1,500 psi).1 To
improve melt uniformity (and mix concentrates), increase melt
temperature, or to get rid of air entrapment (air splay), back pressure
can be increased gradually to as much as 15.5 MPa (2,250 psi).1 High
back pressures can aggravate drooling into the mold and require
additional decompression.
Decompression (Suck Back)
- In general, use very small or no decompression because it tends to pull
air back into the nozzle causing splay in the next shot. Very small
amounts of decompression can be used to reduce drool if needed.
Screw and Barrel Design
- General-purpose screws with compression ratios in the 2.8:1 or 3:1
range and L/D ratios of 18–22:1 have been used successfully. The
transition zone should have a gradual transition (typically 4–6
diameters) so that the high shear heating of a sudden transition is
avoided. Screws should be chosen to be compatible with the hardness
of the barrel material to minimize wear as with any plastic material.
These unfilled materials are generally very mild on screw wear.
Corrosion of barrel and screw parts is not expected with DuraStar
polymers.
Purging
Purging other polymers to DuraStar DS1000/DS1010:
- Purge with clear undried polycarbonate or clear polycarbonate regrind
at 270º–290ºC (518º–555ºF) melt temperature to eliminate the
previous polymer. After an adequate amount of purging, which will
vary depending on the previous polymer molded, the polycarbonate
that is in the barrel of the injection-molding machine can be followed
directly with DuraStar polymer without further purging. Note that
DuraStar polymer may have a yellow cast to it until all polycarbonate
has cleared the injection system.
Purging from DuraStar DS1000/DS1010 to other polymers:
- Purge with acrylics, PS, commercial purging compounds, or the
polymer that follows DuraStar polymer.
Mold Construction
These guidelines can be used to minimize sprue sticking, reduce cycle
time, and widen the processing window.
- Taper to 3º minimum (included angle) on the sprue bushing.
- Orifice size of the sprue bushing where it meets the nozzle should be
4 to 7 mm (5/32 to 9/32 in.) diameter. Larger parts will need orifice
diameters of 7 mm while smaller parts will need only 4 mm.
- Shorten the sprue bushing "L" dimension to less than 75 mm (3 in.)
in length.
- The sprue bushing should have a high polish in the sprue area.
- Increase cooling around the sprue bushing—upper and lower water
line circuits are recommended.
- Good surface contact between the sprue bushing and mold surface.
– Line-on-line interference fit is recommended.
– Surface contact should be on the head of the sprue
bushing as well as the shaft.
- Water line spacing 50 to 64 mm (2 to 2.5 in.) between center lines.
- Air poppets should be offset as far as possible from the center line of
the sprue.
For example, a sprue bushing for a medium size part should have a
length of 75 mm (3 in.) or less and a sprue bushing orifice diameter of
5.5 mm (7/32 in.).
In cases where aggressive molding cycles are desired, substitute a
Performance Products alloy sprue bushing for the steel sprue bushing.
Alloy sprue bushings are fabricated from raw materials that enjoy
significantly better thermal efficiency than traditional steel sprue
bushings.
1Note that this is actual hydraulic pressure not molding
machine gauge pressure.
Important Notice
Material Safety Data Sheets providing safety precautions that should be observed in
handling and storing Eastman products are available online or on request. You should obtain
and review the available material safety information before handling any of these
products. If any materials mentioned are not Eastman products, appropriate industrial
hygiene and other safety precautions recommended by their manufacturers should be observed.
Neither Eastman Chemical Company nor its marketing affiliates shall be responsible for
the use of this information, or of any product, method, or apparatus mentioned, and you must
make your own determination of its suitability and completeness for your own use, for the
protection of the environment, and for the health and safety of your employees and purchasers
of your products. No warranty is made of the merchantability or fitness of any product,
and nothing herein waives any of the Seller's conditions of sale.
DuraStar and Eastman are trademarks of Eastman Chemical Company.
© Eastman Chemical Company, 2000.
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